mx6cuboxi: Allow HDMI and USB keyboard to be stdout/stdin

There are users of Cuboxi and Hummingboard that use these boards without
connecting them to a USB/serial adapter.

Allow such usage by allowing the HDMI port to act as stdout and USB keyboard
as stdin.

The serial console still also works as stdin/stdout.

Signed-off-by: Rabeeh Khoury <[email protected]>
Signed-off-by: Fabio Estevam <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Tested-by: Vagrant Cascadian <[email protected]>